Hematopoiesis and vasculogenesis are interdependent developmental processes involving the formation of blood cells and vascular structures. Although numerous signaling transducers and transcriptional regulators have been implicated in hemato-vascular development, little is known about the early mechanisms that underlie induction and patterning of the mesodermal cells that will be specified along hematopoietic and vascular endothelial lineages. In this project I will examine the potential role of the murine Mix (mMix) homeoprotein in mesoderm induction and early hemato-vascular development. In the early embryo, expression of the mMix gene partially overlaps that of another mesodermal gene, Brachyury (T), in the primitive streak and nascent mesoderm. In ES cell-derived embryoid bodies, mMix and T are coexpressed in a population of mesoderm with hematopoietic and endothelial (hemangioblastic) potential, prior to the appearance of differentiated hematopoietic and endothelial cells. Targeted disruption of the mMix gene results in a collection of defects that include abnormalites in allantoic (umbilical vasculature) and heart morphogenesis. Because mMix is a DNA-binding transcription factor, I hypothesize that mMix may regulate genes required for the induction and/or patterning of some populations of hemato-vasculogenic mesoderm. A combination of genetic, cell biological, biochemical and bioinformatics approaches will be employed to address the following three aims: (1) To examine the ability of Mix to regulate induction and/or patterning of hematopoietic and vascular mesoderm in differentiating ES cells; (2) To characterize the transcriptional properties of the mouse Mix protein; and (3) To identify potential target genes of mMix using functional genomics. These studies will uncover early regulatory events in hematopoietic and vascular development and will, therefore provide novel insights into the formation, self-renewal, and differentiation of hematopoietic and endothelial stem/progenitor cells. They may ultimately facilitate the development of new stem/progenitor cell therapies for treatment of hematologic and other malignancies and find important applications in regenerative medicine.